Static random access memories (SRAM) are commonly used in integrated circuits. Embedded SRAM is particularly popular in high speed communication, image processing and system-on-chip (SOC) applications. SRAM cells have the advantageous feature of holding data without requiring a refresh. Typically, a SRAM cell includes two pass-gate transistors, through which a bit can be read from or written into the SRAM cell. This type of SRAM cell is referred to as a single port SRAM cell. Another type of SRAM cell is referred to as a dual port SRAM cell, which includes four pass-gate transistors. With two ports, the bit stored in the SRAM cell can be read from port-A and port-B simultaneously. This allows for parallel operations by different applications. Moreover, if a first SRAM cell and a second SRAM cell are in a same column or a same row, a read operation from the first SRAM cell can also be performed simultaneously with a write operation to the second SRAM cell.
The lowest VDD voltage (positive power supply voltage) at which an SRAM bit cell may function is referred to as Vccmin. Having a low cell VDD near Vccmin reduces leakage current and also reduces the incidence of read flips. But having a high cell VDD improves the probability of successful write operations. Therefore, the Vccmin is limited by the write operation.